1. Field of the Invention
This invention pertains to an improved cyclic steam stimulation method for the recovery of oil from subterranean viscous oil-containing formations. More particularly, it relates to a cyclic steam stimulation method for the recovery of oil from subterranean viscous oil-containing formations utilizing a selected completion interval to induce maximum visbreaking of the viscous oil within the formation.
2. Background of the Invention
Steam has been used in many different methods for the recovery of oil from subterranean, viscous oil-containing formations. The two most basic processes using steam for the recovery of oil includes a "steam drive" process and "huff and puff" steam process. Steam drive involves injecting steam through an injection well into a formation. Upon entering the formation, the heat transferred to the formation by the steam lowers the viscosity of the formation oil, thereby improving its mobility. In addition, the continued injection of the steam provides the drive to displace the oil toward a production well from which it is produced. Huff and puff involves injecting steam into a formation through an injection well, stopping the injection of steam, permitting the formation to soak and then back producing oil through the original injection well.
Heavy or viscous oils go through significant visbreaking upon heating at temperatures normally above 400.degree. F. Such reduction in viscosity significantly enhances recovery during a thermal operation and improves the quality of heavy oils.
In copending application Ser. No. 320,236, filed Nov. 12, 1981 to W. R. Shu et al, now abandoned, there is disclosed a method to induce in-situ visbreaking of heavy oils by injecting a hot heat-transfer fluid such as steam in a cyclic operation. In the later method, it was found that visbreaking was greater in a formation with no underlying water zone and with a large completion interval during cyclic steam stimulation.
We have found that the cyclic steam stimulation method can be improved to induce maximum visbreaking of viscous oils in-situ for a formation with or without a significant underlying water layer if the steam is injected through a selected completion interval to control heat losses to the over and under strata.